The present invention relates to a method and apparatus for detecting myocardial ischemia.
Myocardial ischemia can be defined as a decreased supply of blood to the heart, and more precisely as an imbalance between the myocardial oxygen supply and demand. In most clinical situations, the reason for this imbalance is inadequate perfusion (blood injection) of the myocard (muscle tissue of the heart) due to obstructions or stenosis (a narrowing) of the coronary arteries (the arteries that supply blood to the heart). The ischemia can last only a few seconds or it can persist for minutes or even hours, causing transient or permanent damage to the heart muscle (myocardial infarction). Myocardial ischemia is usually accompanied by chest pain (angina). In some cases, however, it is not accompanied by pain, or the subject is not aware of the pain, for example, when the subject is unconscious, and therefore detection of the ischemia must be made by objective methods rather than by relying on complaints of the subject.
The most commonly used objective criteria for ischemia detection and monitoring are the electrocardiographic (ECG) changes at rest or during effort testing. Ischemia can be demonstrated by the elevation or depression of the S-T segment, by inversion or other changes in T-waves, or by changes in the shape or width of the QRS complex. However, sometimes electrocardiographic changes are not detected because the appropriate electrocardiograph lead (of the 12 commonly used leads) is not being monitored. At other times, the electrocardiograph is too sensitive and reflects changes that have no real significance.
For these reasons, methods other than use of the ECG, are employed to detect myocardial ischemia. These other methods include:
a. Hemodynamic Changes Associated with Ischemia--subject's blood pressure. Blood pressure can therefore also be used for continuously monitoring for myocardial ischemia. This method is commonly used in operating rooms; and it is good cardiac anesthesia practice to prevent increases and decreases of blood pressure as much as possible. However, changes in blood pressure can result from pain or from other reasons; and therefore, changes in blood pressure alone are unreliable as the primary indicator of ischemia.
Another commonly used hemodynamic parameter is the pressure in the left atrium. This parameter can be monitored indirectly, for example, by using a Swann-Ganz catheter which measures the pulmonary-capillary wedge pressure that is usually equal to the left atrial pressure. Left atrial pressure can also be measured directly after open heart procedures through a catheter introduced into the left atrium. In catheterization laboratories, the left ventricular end diastolic pressure (LVEDP) can be measured through a catheter introduced through the aorta. Changes in left atrial pressure usually reflect changes in LVEDP, and ischemia is usually associated with increased LVEDP. Because of the highly invasive nature of the pressure measurements of the left atrium, pulmonary-capillary wedge, or of the left ventricle, these methods are used only in special situations. It is also important to note that ischemia is not always associated with increased LVEDP.
b. Two-Dimensional Echocardiography--Important changes in ventricular wall motions or in ventricular dimensions are associated with ischemia. Two-dimensional echocardiography, using external transducers, can detect increased left ventricular end diastolic and end systolic volume. A trans-esophageal echocardiographic transducer allows continuous detection and monitoring of changes in ventricular wall motion, and therefore also enables monitoring of ischemia.
c. Radionuclide Ventriculography--Injection of radioactive marker (Tc-99n phyrophosphate stanus) that adheres to the myocardial muscle provides a method for monitoring changes in ventricular wall motion, and therefore also enables detection of ischemia. This method for the non-invasive detection of ischemia is used during rest and effort tests.
d. Thalium 201H perfusion scans provide a further method for the selective and non-invasive monitoring of the blood supply to the heart. Although radionuclide ventriculography and Thalium perfusion scans can detect ischemia, they both involve large and expensive instruments, and therefore these methods are not commonly used for monitoring of ischemia.
An object of the present invention is therefore to provide a new method and apparatus for detecting myocardial ischemia.